It is well-known in the state of the art to manufacture semiconductor devices, for example silicon integrated circuits, which comprise a buried layer. According to the application, the buried layer must be operable to be electrically contacted through an overlaying layer.
A bipolar transistor having a collector formed by a buried layer is an example. The buried layer is usually covered by an epitaxially grown layer of silicon separating the collector from the base of the transistor. The epitaxially grown silicon layer is limited laterally by trenches to form a silicon well.
The collector formed by the buried layer is to be electrically contacted on the upper surface of the device. Therefore, a deep contact providing an electrical connection between the buried layer and the upper surface is needed. This deep contact must have a low series resistance. In the state of the art, part of the silicon well is used to provide electrical contact between the buried collector and the upper surface of the device.
In order to obtain a low resistance, the series resistance of the deep contact may be reduced by reducing the depth at which the buried layer is buried or in other words by reducing the thickness of the epitaxial layer deposited on top of the buried layer. But a required minimal breakdown voltage for the vertical junction breakdown behavior may require a minimal epitaxial thickness. Therefore, the epitaxial thickness cannot always be reduced sufficiently. An example for a required thickness in BICMOS technology (bipolar and CMOS transistors integrated in the same device) lies at about 5 μm.
Another possibility for reducing the series resistance is to use a larger contact area, which means a greater circuit surface needed for a transistor. This is contrary to the advancements in technology in terms of scaling vertical and lateral dimensions.
The resistivity of silicon can be influenced by doping the silicon. Well known dopants comprise arsenic and boron, but other dopants may be used as well. With an increasing dopant concentration, the resistivity decreases. The doping must be uniform in order to have a low series resistance. Doping a thick epitaxial silicon layer requires high implants of the order of several MeV implants to get uniform contact diffusion doping across the epitaxial depth. As there is only a limited lateral space, only a limited amount of doping can be implanted without generating massive implant damage and of course without residual implant damage even after optimized anneal sequences.
Thus, in the state of the art, the further lateral scaling of bipolar transistors is limited by the increasing contact resistance and/or increasing the vertical breakdown voltage is limited by the contact resistance.